Rise and fall of water blisters offers glimpse beneath Greenland's thick
ice sheet

Date:
August 23, 2021
Source:
Princeton University
Summary:
A study found that as meltwater lakes on the surface of Greenland's
ice sheet rapidly drain, they create water blisters between the
ice and the bedrock that scientists could use to understand the
hydrological network below Greenland's thick inland ice sheet. These
networks could affect the stability of the ice sheet as Earth's
climate warms.



FULL STORY ========================================================================== Water "blisters" trapped beneath the thick interior of Greenland's
ice sheet could provide critical insight into the hydrological network
coursing deep below Earth's second largest body of ice -- and how it
might be destabilized by climate change, according to a new study.


==========================================================================
Each year, thousands of natural meltwater lakes form on the surface of
the ice sheet's high-elevation interior, where ice can be more than a
half-mile thick.

As these lakes drain, they form large water-filled cavities between the
ice and the bedrock.

By combining field observations with mathematical models and laboratory experiments, Princeton University-led researchers discovered that these blisters push the surface of the ice upward, then cause it to gradually
drop down as the water permeates into the subglacial drainage system,
according to a report in the journal Nature Communications.

The team shows for the first time that the rise and fall of the ice sheet caused by rapid lake drainages can be used to estimate a property known as transmissivity, which characterizes the efficiency of the water networks
that form between the ice and the bedrock. Lake drainage presents a new
tool for gauging transmissivity beneath inland regions of the ice sheet,
where transmissivity is otherwise difficult to measure, the researchers reported.

They found that transmissivity can increase by as much as two orders of magnitude during Greenland's summer melt season.

The findings could shed light on how climate change will affect
Greenland's vast frozen interior as the planet warms and surface melting increases, said first author Ching-Yao Lai, an assistant professor of geosciences and atmospheric and oceanic sciences at Princeton. Water from surface melting can act as a lubricant, she said, causing the glacier
to slide more easily across the bedrock.

Existing research has shown that a major way for surface melting to impact
the stability of the Greenland ice sheet is by meltwater lubricating the ice-sheet bed, Lai said. The majority of these studies, however, have
focused on low- elevation areas where the ice sheet is thinner. Previous studies also have suggested that increased surface melt could accelerate
the velocity of the high-elevation, interior ice sheet, but these findings
are based on computational models, rather than observations, Lai said.



==========================================================================
The paper in Nature Communications provides a rare, observation-based
glimpse into the largely inaccessible water networks underlying
Greenland's high- elevation ice sheet. The study was supported by
Princeton's High Meadows Environmental Institute (HMEI) and the HMEI
Carbon Mitigation Initiative.

"We know that as the climate warms in the future, the surface melt zone
can expand and migrate to higher elevations than currently observed.A big question that remains to be answered, however, is how much transmissivity
can increase further inland," said Lai, who is an associated faculty
member in HMEI.

"A potential impact is that the link between surface melt and subglacial
water- network development could be activated not only at lower
elevations, as currently observed, but also at higher elevations," she
said. "More observations of seasonal changes of subglacial transmissivity
in response to surface melting would be needed to really understand
what would happen when melt migrates to higher elevation regions."
Co-authors of the paper from Princeton are HMEI associated faculty
member Howard Stone, Princeton's Donald R. Dixon '69 and Elizabeth
W. Dixon Professor of Mechanical and Aerospace Engineering and chair
of mechanical and aerospace engineering, and Danielle Chase, a graduate
student in Stone's Complex Fluids Group.

The study co-authors also included Laura Stevens, an associate professor
of climate and earth surface processes at the University of Oxford who has extensive experience studying lake drainages and ice dynamics. Stevens
helped collect the field observations in Greenland with co-authors
Mark Behn, an associate professor of earth and environmental sciences
at Boston College, and Sarah Das, an associate scientist at Woods Hole Oceanographic Institution.

Timothy Creyts from the Lamont-Doherty Earth Observatory at Columbia
University also is a co-author on the study.



==========================================================================
The researchers used GPS data and field observations of five lake-drainage events that occurred between 2006-12 to estimate drainage volume and
to observe surface displacements caused by lake drainage and subsequent
blister formation.

"We observed in the GPS data a wide range of ice-sheet uplift relaxation
times following the five drainage events," Stevens said. "We had an
inkling that this spread in relaxation times might be indicative of some characteristic of the subglacial drainage system. Our understanding
was significantly improved as this collaboration between researchers
with expertise in observational, theoretical and experimental approaches catalyzed." Chase -- who received a HMEI Walbridge Fund Graduate Award to study fluid- driven fracturing -- then designed a series of experiments
using a type of silicone that mimics the deformable ice overtop a porous material that represents the bedrock. She injected fluid between the
deformable sheet and the porous substrate, observing the time it took for
a blister to form and then drain into the porous substrate. Working with
Stone and Lai, Chase also developed a mathematical model that explains
the physics that govern the surface uplift and relaxation due to water
blister formation. Her work is the topic of a paper recently accepted
by the journal Physical Review Fluids.

"Experiments can be helpful because, in the lab, we can control and
measure all the parameters in the system, which allowed us to test our
model," Chase said.

"We also can choose ideal materials. The system is small enough to be
held in one hand and the material is transparent, so we were able to
directly observe the shape of the blister and the drainage into the
porous substrate over time." The study is unique for using laboratory experiments to investigate natural processes such as blister formation
that are difficult to analyze in the field where researchers cannot
control the parameters.

"It is valuable to have laboratory models to better understand the
mechanisms behind the complex shape changes that occur in nature,"
Stone said. "Here, the laboratory experiments captured the main
mechanical features observed in the field and helped us understand
the relaxation of the ice sheet as water drains along the glacial bed." ========================================================================== Story Source: Materials provided by Princeton_University. Original
written by Morgan Kelly.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ching-Yao Lai, Laura A. Stevens, Danielle L. Chase, Timothy
T. Creyts,
Mark D. Behn, Sarah B. Das, Howard A. Stone. Hydraulic
transmissivity inferred from ice-sheet relaxation following
Greenland supraglacial lake drainages. Nature Communications,
2021; 12 (1) DOI: 10.1038/s41467-021- 24186-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210823125717.htm

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